As a critical chemical agent for implementation of the discharge printing process, the function of a discharge agent is usually to be subjected to a chemical reaction (usually a reduction reaction) with a ground color dye of a printed part during ageing (treatment under certain hygrothermal condition) after printing, so as to damage the chromophoric system of the ground color dye for decoloration.
At present, discharge agents commonly used in the printing and dyeing industry are formaldehyde sulfoxylate type reductants, such as solid rongalite C (sodium formaldehyde sulfoxylate) and decrolin (zinc formaldehyde sulfoxylate), pasty leucotrope H (calcium formaldehyde sulfoxylate) and a liquid discharge agent Rongalit ST Liq. But these discharge agents have the following problems: (1) free formaldehyde is released, printed products cannot meet the requirement of ecological textiles, particularly in the pigment discharge printing, it is more difficult to remove the residual formaldehyde-contained discharge agent due to the application of a binder, and severe washing largely weakens the meaning of water-free printing for pigment printing; (2) the commonly used discharge agents are easy to dissolve and damage during preparation and storage of discharge paste to cause large consumption of the discharge agents and high cost; and (3) the commonly used discharge agents all contain metal ions, even heavy metal ions, but the synthetic thickeners used during pigment printing are generally not resistant to the (heavy) metal ions, causing the implementation of the pigment discharge printing process difficult.
Thiourea dioxide, also called thiourea dioxide with molecular formula of (NH2)2CSO2, is stable at room temperature with neither oxidability nor reducibility, and dissolves and separates sulfoxylate with strong reducibility when heated or in alkaline conditions. And meanwhile, the thiourea dioxide does not release formaldehyde and does not contain heavy metal ions, so it is an environmentally-friendly chemical medicine. In conclusion, by using the thiourea dioxide as the discharge agent for discharge printing, the above three problems which exist in formaldehyde sulfoxylate type reductants commonly used in printing and dyeing mills can be solved. However, during practical production and practice, the thiourea dioxide also has some problems when used as the discharge agent for printing: (1) under normal conditions, the thiourea dioxide is white and odorless crystalline grain with low solubility (the solubility is only 26.7 g/L in water at the temperature of 20° C.), and the discharge paste for discharge printing consumes a small amount of water during preparation, as a result, the thiourea dioxide used as the discharge agent during discharge printing cannot be fully dissolved and uniformly dispersed in the discharge paste, instead, it exists in the paste non-uniformly in the form of granular crystal, so the net permeability of the thiourea dioxide and the reduction effectiveness to the ground color dye of the printed part are affected; and, (2) in order to make the discharge agent thiourea dioxide better dissolved and dispersed in the discharge paste, the thiourea dioxide may be mechanically ground in advance. Researches show that, compared with a dry grinding process, the discharge agent thiourea dioxide has smaller particle, better dispersibility and better discharge effect after subjected to wet grinding by using water as medium. However, during the storage of the discharge agent thiourea dioxide subjected to wet grinding, the thiourea dioxide is easy to dissolve and is low in stability due to absorption of and contact to oxygen in air (oxygen may be dissolved in water). The above problems greatly restrain the popularization and application of the thiourea dioxide in printing and dyeing enterprises as discharge agent for printing.